electron - molecule collisions

Description

we now use rough guesses - this paper quantifies them

Goldsmith.P17Electron-Excitation-of-High-Dipole (article)
Electron Excitation of High Dipole Moment Molecules Re-examined
Goldsmith, P. F. & Kauffmann, J.
2017, \apj, 841, 25
ABSTRACT: Emission from high-dipole moment molecules such as HCN allows determination of the density
in molecular clouds, and is often considered to trace the dense'' gas available for star formation. We assess
the importance of electron excitation in various environments. The ratio of the rate coefficients for
electrons and H2 molecules, ~=105 for HCN, yields the requirements for electron excitation to be of
practical importance if n({{{H}}}2)<=slant {10}5.5 {{cm}}-3 and X({{{e}}}-)>=slant {10}-5, where the
numerical factors reflect the critical values {n}{{c}}({{{H}}}2) and {X}* ({{{e}}}-). This indicates that in
regions where a large fraction of carbon is ionized, X({{{e}}}-) will be large enough to make electron
excitation significant. The situation is in general similar for other high-density tracers,'' including HCO+, CN,
and CS. But there are significant differences in the critical electron fractional abundance, {X}* ({{{e}}}-),
defined by the value required for equal effect from collisions with H2 and e-. Electron excitation is, for
example, unimportant for CO and C+. Electron excitation may be responsible for the surprisingly large
spatial extent of emission from dense gas tracers in some molecular clouds. The enhanced estimates for
HCN abundances and HCN/CO and HCN/HCO+ ratios observed in the nuclear regions of luminous galaxies
may be in part a result of electron excitation of high dipole moment tracers. The importance of electron
excitation will depend on detailed models of the chemistry, which may well be non-steady state and non-
static.
KEYWORDS: ISM: molecules, molecular processes, photon-dominated region: PDR
NOTE:
LOCAL FILE : ~/Dropbox/literature/molecules/collisions/electronCollisions/Goldsmith.P17Electron-Excitation-of-High-Dipole.pdf
REMOTE URL : http://dx.doi.org/10.3847/1538-4357/aa6f12
ADS URL : http://adsabs.harvard.edu/abs/2017ApJ...841...25G